Aromatic streams, which may comprise one or more of benzene, toluene and xylenes (BTX), are used as feedstocks in various petrochemical processes. By way of example, paraxylene obtained from such streams are useful in the production of polyester fibers and films. It is well-known that such streams, derived from processes such as naphtha reforming and thermal cracking (pyrolysis), generally contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes, and that these contaminants must be removed before subsequent processing of the aromatic streams. Zeolites including those from the MWW family of zeolites can remove olefinic compounds from such aromatic streams at least in part by alkylating aromatic compounds with the olefins to form heavy aromatics (C9+ aromatic hydrocarbons) that can, in turn, be removed easily, for instance, by fractionation. See, for example, U.S. Pat. Nos. 6,368,496; 7,517,824; 7,731,839; 7,744,750; 8,048,295; 8,057,664; 8,216,450; 8,227,654; 8,329,971; and 8,344,200.
Less well-known is that in the production of paraxylene by contact of toluene and/or benzene with an alkylating agent such as methanol and/or dimethylether, in the presence of solid acid catalysts, impurities such as oxygenates are produced in side reactions. The product of such alkylation reaction is typically a paraxylene-rich aromatic hydrocarbon stream. The term paraxylene-rich (or “para-rich”) means that paraxylene is present in amounts greater than equilibrium amounts, based on total xylenes, for instance greater than 23 mol %. This is a highly valuable feedstream, because paraxylene is much more valuable relative to its isomers. Since many of the known methods of purification of aromatic feedstreams have the drawback of isomerizing xylenes, and isomerization of a paraxylene-rich aromatic hydrocarbon stream towards equilibrium concentration is completely antithetical to the whole purpose of such alkylation reactions, purification of such product streams is any area of intense research.
In this regard U.S. Pat. No. 8,252,967 teaches such oxygenates may be removed from the aromatic hydrocarbon product of the aforementioned alkylation reaction by use of crystallization technology.
In U.S. Patent Publication No. 2012-0316375 the concentration of phenolic impurities in a xylene stream produced by the aforementioned alkylation reaction can be reduced to trace levels, such as below 0.1 ppmw, by one or more washing treatments with an aqueous solution of a base (caustic).
U.S. Patent Publication No. 2013-0253245 is directed to a process including the aforementioned alkylation reaction, the improvement comprising: (a) determining the amount of at least one oxygenate co-produced in the paraxylene-enriched product; (b) separating said product into separate streams including: (i) one or more streams comprising said unreacted components and co-produced oxygenates; (ii) at least one stream comprising paraxylene; (iii) at least one stream comprising C9+ aromatics, if present; (iv) at least one stream comprising light gases, if present; (c) recycling at least one stream selected from unreacted components and co-produced oxygenates, whereby said at least one of these recycled streams combines with the alkylation reactor feed; (d) determining the amount of said at least one oxygenate in said feed, including said recycle; (e) controlling reactor conditions, step (b) and step (c) so that the amount determined in step (a) is less than or equal to the amount determined in step (d).
Recently there has been described a process for the purification of an aromatic hydrocarbon stream containing phenol and greater than equilibrium amounts of paraxylene, comprising contact of said aromatic hydrocarbon stream with an adsorbent selective for the absorption of phenol relative to xylenes, to provide a product stream having lower concentration of phenol than said aromatic hydrocarbon stream. Suitable materials used to remove phenol from the process stream include alumina, silica, molecular sieves, zeolites, basic organic resins, and mixtures thereof. (U.S. Patent Publication No. 2013-0324780.)
Other relevant prior art includes U.S. Pat. No. 6,555,611, teaching an absorbent for absorbing aromatic hydroxyl compounds, said absorbent comprising composite metal oxide solid solution particles or hydrotalcite-like composite metal hydroxide particles; JP 5639025A, teaching removal of phenols from organic matter by contact with a polyvinyl pyridine resin; CN 1253937A, teaching using silica gel to remove phenol-like compounds from styrene monomers; U.S. Pat. No. 3,409,691, teaching removal of phenol by the use of macroporous ion exchange resin; U.S. Pat. No. 4,064,042, teaching separating an organic component such as phenol from blood by contact with a macroporous synthetic polymer; U.S. Pat. No. 5,218,132, teaching removal of aromatic impurities such as phenol by contact with a material including a smectite mineral; and JP 7215901A, teaching contact of a phenol-containing non-aqueous solvent with an acrylic weak basic ion exchange resin.
There are other sources of oxygenate impurities in xylene feedstocks. For instance, such feedstocks often contact oxygen during transport by ships and may contain unacceptably high levels of oxygenated species that could potentially disrupt downstream processing, for example, poisoning of adsorptive separation units such as Parex™ and Eluxyl™ adsorptive separation units, as well as poisoning of catalysts in the several steps required to convert paraxylene into polyesters and other useful derivatives.
Despite a plethora of means to purify various aromatic hydrocarbon streams comprising xylenes, the industry is not satisfied with the results and research into new methods is intense. There is still a need, specifically, for a simple and effective way of purifying paraxylene-rich aromatic hydrocarbon streams to remove impurities including olefins and oxygenates.